Adding an option group using AWS CloudFormation

I'm building a CloudFormation template that will create a SQL Server database (using RDS) with the Multi-AZ option (so it maintains a synchronous standby replica in a different Availability Zone).

However, to make this work, I need to associated the database instance with an option group that has the Mirroring option. Haven't been able to find anywhere how to create an option group with a CloudFormation template.

How do I create an option group in a CloudFormation template?